Pasteurization is the process whereby microbiological organisms are inactivated to promote stability and improve the shelf life of beer. Pasteurization is accomplished by heating the product to approximately 140 .degree. F. for a predetermined period of time. Under-pasteurization may result in less than complete destruction of organisms. Over-pasteurization may result in flavor changes.
Pasteurization has progressed from simple but labor intensive methods to completely automated methods. Newer methods generally utilize either bulk pasteurization or tunnel pasteurizers.
Bulk pasteurization is the process of pasteurizing beer before it is packaged in a container. This method requires sterile filling equipment and does not prevent biological contamination from being introduced by the container.
Tunnel pasteurizers utilize the principle of conveying the package through heating and cooling zones, during which time the packages are subjected to sprays of water. The temperatures of the water sprays are predetermined and controlled such that each successive zone further heats the product up to a determined holding temperature. After the product is subjected to a constant temperature for a specific period of time, it proceeds through a series of cooling zones during which the product temperature is systematically lowered to a desired beer out temperature.
The most commonly used method of conveyance through a tunnel pasteurizer is by means of a walking beam. Walking beams utilize grate strips which rise and advance a nominal distance before lowering and depositing the product on a separate series of grate strips. This second series of grate strips can rise and advance the product while the first series of grate strips is returning to its original position or it can remain stationary while the first series of grate strips is lowering and returning to its original position prior to beginning a new cycle.
Water spray systems, including volumes, spray patterns and water distribution systems will vary depending on the pasteurizer manufacturer. Heating is required to elevate product temperature to the proper pasteurization temperature and is typically accomplished by using steam or hot water in either direct injection or indirect heating (heat exchanger) mode. Cooling is required to lower product temperature to an acceptable level before labeling, packing and storing. Cooling is typically accomplished by the direct injection of a cooling medium. This medium can be water utilized at the "as received" temperature or water which is recycled and cooled mechanically in a cooling tower or water chiller.
Early pasteurizers made little or no attempt to conserve or reuse the energy expended in heating and cooling the product during the pasteurization process. Initial attempts at conserving resources and energy resulted in energy regeneration. Regeneration couples a cold zone and a hot zone into a closed loop called a regenerative pair. The spray water is cooled as it heats up cool product before being pumped to its regenerative pair. In this paired zone, the cooled spray is heated up as it cools the hot product before being pumped to the cold zone. This heat transfer system is called regeneration. Pasteurizers can be provided with as many regenerative pairs as desired. Economic justification becomes difficult at five or more regenerative pairs.
Tunnel pasteurizers inherently possess advantages and disadvantages. The pasteurizers perform a relatively simple function in a complex manner. Heating and cooling containers is simple. Regeneration of energy and the associated heat balances are complex.
The following is a list of problems associated with tunnel pasteurizers and a short discussion of each:
(a) Under-pasteurization--Under-pasteurization is a relative term which is dependent on the biological condition of the product being packaged, the cleanliness of the container, and the level of assurance required by the manufacturer. While 2-3 pasteurization units (PU's) is generally adequate in destroying microbiological contaminants, most brewers establish a minimum PU limit between 5-10 PU. Underpasteurization most often occurs unknowingly: i.e., failures in the heating controls, product advanced too quickly through the heating and holding zones or faulty spray water distribution.
(b) Over-pasteurization--Over-pasteurization occurs more frequently than under-pasteurization since brewers would rather err on the high side, and control equipment is designed to fail accordingly. Downsteam delays force the pasteurizer to stop. When the pasteurizer stops, the product which has been heated to approximately 140.degree. F. continues to pasteurize and will continue to do so until the product has been cooled to nominally 120.degree. F. e.g. a ten minute delay at 145.degree. F. will add over 25 PU to the product. Failure of heating controls and faulty calibration also can cause over-pasteurization.
(c) Water Consumption And Waste--Typically, pasteurizers with three or less regeneration pairs require continuous additions of cooling water. When utilities were cheap, this water was normally injected where needed in the pasteurizer, allowed to overflow from the machine, and was sewered. Typically today, this overflowing water is collected, recooled and recycled to the pasteurizers. To insure that water is maintained in each sump at an adequate level, manual make-up water valves are often left open. This practice caused water to eventually overflow to a sewer or a reclaim system where it may or may not have been excess.
(d) Heat Waste--As discussed in (c) above, excess water overflowing at a higher temperature than when injected becomes a source of heat waste. Normally, a "balanced" pasteurizer will require continuous addition of heating energy. When no product is in the hot side of a regenerative pair, more heating energy must be provided to compensate for the heating normally accomplished by the hot product which is being cooled. This extra heating is also required if less product is in the hot regenerative pair than is in the cold regenerative pair. This condition is an "unbalanced" machine.
When the pasteurizer is being filled with product (run-in) at the beginning of a day or shift, the hot end is completely empty and the heating normally provided by the hot product must be supplied by steam or heated water. This extra heat required during run-in is eventually lost and is considered a "necessary" waste of heating energy.
(e) Cooling Energy Waste--As stated in (d) heating energy is wasted during run-in. Conversely, when it is time to empty the machine, large amounts of cooling energy are required to replace the cool product which normally is entering the pasteurizer. This cooling requirement is also eventually lost and is considered a "necessary" waste of cooling energy.
(f) Simultaneous Heating And Cooling Energy Waste--If the supply of product to the pasteurizer is delayed the beginning of a run-out will occur. When product supply resumes, a gap or space void of product will have been formed in the pasteurizer. A series of such gaps frequently results in external heating and cooling being provided simultaneously in a regeneration pair. This situation is a common source of cooling and heating energy waste.
(g) Beer Waste And Losses--The primary causes of beer waste and losses are over and under-pasteurization, tipped and broken containers. The causes of the first two have already been discussed. Tipped containers caused by the previously mentioned gaps cause machine and conveyor jams and are thus damaged and wasted. An indirect result of broken or leaking containers is organic contamination of the cooling source which will result in cross-contamination of one or more pasteurizers. Broken bottles normally result from thermal shock which is caused when hot or cold product enters a successive zone that is either too cold or too hot, respectively. The temperature difference between the spray water and the outer surface bottle when excessive causes thermal shock. Generally, if water spray temperature is 80.degree. F. hotter or 40.degree. F. cooler than the bottle skin temperature, thermal shock will result.